Introduction

On Sunday March 3, 1985 at 7:47 p.m. a historic earthquake occurred off the coast of central Chile. The epicenter of the quake was located in the Pacific Ocean approximately 25km offshore, about 120km due west of Santiago and 80km southwest of Vina de Mar, with an estimated focal depth of 33km. It was documented as a magnitude 7.8, however, it was comprised of two separate events separated only by ten seconds. This calculated magnitude makes the 1985 Chile Earthquake one of the largest seismic events worldwide during the 20th century. Extensive damages and casualties were reported in the central region of Chile. Approximately 66,000 homes were destroyed, another some 127,000 were damaged, and an estimated 1 million people were left homeless. Approximately 200 people were killed and another 2,000 were injured. The damaged area was very large, extending as far south as Curico, 200km from the epicenter, and to Santiago, 120km away. The total cost of damages was estimated around 1 billion U.S. dollars. (Wood, 1987)(Booth 1985)

Chile Background

Seismicity of Chile

The boundary between the Nazva and South American Plates parallels the Chilean coast. The intense seismic activity of the region is a direct result of the crustal movements and interaction along this fault line. As described in Figure-1, the source of this activity is most likely due to eastward subduction of the Nazca plate along the coast at a rate of approximately 10cm per year and due to two main mechanisms: 1) Interplate events at the interface of the descending slab and the South American Plate and 2) Interplate events located 20-30km inside the descending slab. It is believed that large events of magnitude 8 would likely result from the first mechanism and up to 7.5 expected from the second. The slope of the Nazca plate is not constant and alternating steep and gentle slopes are realized. High volcanic activity is coupled with the presence of the steeper inclinations. Vina Del Mar is located on one of the boundaries between two of these different zones. (Wood, 1987)

Previous Earthquakes/ Seismic activity

Large seismic events are not new to Chile. Records indicate similarly large recurring seismic events dating back to the arrival of Europeans back in the 1600’s. This recurrence of full plate rupture and high magnitude events consistently occurs in approximately 80 year intervals . The earthquake of 1985 fits into this trend, with the addition of severe earthquakes in 1575, 1647, 1730, 1822, 1906. Additional events caused serious damage in the midst of these events as well. One hypothesis even suggests another large seismic event is likely to occur in the near future south of the 1985 earthquake. (Wood, 1987)

Geography

Geographically Chile is divided into twelve regions with the addition of the Metropolitan Area of Santiago. The cities of Vina del mar and Valsparaiso are located in the fifth region. The population is largely concentrated in the center of the country and approximately one-half of that population is located in the fifth region and the metropolitan area. These include a large concentration of engineered structures, which amounted to 76% of total constructed area in 1981 and 57% in 1984. (characteristics-Wight)The resort city of Vina del Mar is of particular interest from an engineering standpoint because approximately 400 moderate-rise reinforced concrete buildings and a strong-motion instrument were located in the city. (Wood, 1987)

Structural Engineering Practice

Building construction in Chile typically has been limited to low-rise structures. In 1984 statistics indicated that 90% of the total constructed area consisted of one and two story buildings. Only 1% is dedicated to those having nine or more stories. Most buildings are constructed using unreinforced and reinforced masonry. Steel construction is limited due to the high cost of structural steel and low availability. In fact heavy structural steel sections are not even rolled in the country. It is most commonly found on bridge and industrial projects, Prestressed and precast concrete construction is not typical for buildings, but is use for bridges and underground parking structures. In Vina del Mar all buildings having five or more stories were constructed from reinforced concrete columns and structural walls to properly resist lateral loads such as earthquake and wind. The structural walls in these taller buildings tend to occupy a large portion of the footprint of the buildings, typically 6%. (Wood, 1987, Riddell, 1987)

Design Concepts

Buildings in Chile are designed based on a philosophy with respect to acceptable damage and safety similar to that of the U.S. Minor damage is expected and acceptable during minor earthquakes, however structural failures should be avoided during severe earthquakes. The difference between Chile and the U.S. is in the expected earthquake intensities. Earthquakes with a magnitude between 6.5 and 7.0 are considered minor in Chile with 7.5 corresponding to a moderate earthquake. The frequency of strong earthquakes in Chile has resulted in building construction and designs differing from the U.S. in that they are typically designed as “rigid” structures. As seen in the taller buildings of Vina Del Mar this rigidity is accomplished through the large proportions of structural walls to resist lateral loads due to seismic events. (Riddell, 1987)

Development of Seismic Design Provisions in Chile

Sesimic design regulations were first initiated in 1928 as a result of the Talca earthquake on December 1. The first official regulation relating to seismic design provisions was La Ordenanza General de Contrucciones y Urbanizacian, which was adopted in 1935. As of 1985 the current seismic code was Calculo Antisismico de Edificios, adopted in 1972. Major developments took place to result in this modern code. (Riddell, 1987)

Building height limits for multiple construction types were established in a general ordinance in 1939 as follows: 1) 12 stories (40m) for Reinforced Concrete, 2) 13 stories (40m) for Structural Steel, 3) 3 stories (12m) for Timber, 4) 2 stories (8m) for Masonry, and 5) 1 story (3.5m) for adobe. This ordinance did not permit unreinforced masonry construction and included special requirements for adobe construction. This ordinance also established a base shear coefficient for seismic design and requirements with respect to natural periods of vibrations for buildings. Calculated natural frequencies between 1 and 2 were not permitted. (Riddell, 1987)

The 1939 ordinance, however, led to higher building costs, which experienced opposition the AEC community. A government appointed committee revisited these requirements and produced a revision in 1945 containing less severe seismic requirements. This included the elimination of height restrictions for reinforced concrete and structural steel buildings. This general ordinance also established a simple way to calculate total lateral seismic forces for a building as well as the distributions of forces along the height of the building for lateral force resisting design. This is similar in design methodology to the current U.S. design codes. The total lateral force is defined as:

where,
P = the total weight of the building plus a percentage of the live load
C = the seismic coefficient depending on the period of the building, soil conditions, and foundation type.

Forces are distributed over the height of the building to individual stories as follows:where,K= story levelFk= lateral force at story levelPk= weight of story kZk= height of story kThe seismic design provisions as of 1985 are very similar to the General Ordinance of 1949 With minor additions and modifications including the 1972 Nch 433 building code adopted in 1972. (Riddell, 1987)

Damage Assessment/ Lessons Learned

As described above the damage due to the 1985 earthquake was significant and widespread extending over 200km from the epicenter, which was of the coast of Chile. The 1 billion dollars of damages was equivalent to approximately 10% of the annual Gross Domestic Product of Chile.
The main seismic event tested the integrity of structural designs in particularly in the coastal towns of Valparaiso, Vina Del Mar, San Antonio, and the capital Santiago. Although several engineered structures were damaged, the overall performance of engineered structures subjected to such large ground motions should be considered more than adequate. Non engineered and older structures such as adobe construction were heavily damaged throughout the region. (Algermissen, 1985)

Construction Types

Damage varied based on the type of constructions. In general most adobe buildings near the epicenter experienced notable damage and exhibited partial or total collapse. Wood residential and low-rise masonry with reinforced confined concrete elements buildings typically performed well. Steel structures, which are uncommon, did not sustain notable damage. Damage to industrial facilities and bridges were mostly light and widespread in the region. Again, most reinforced concrete structures sustained little or no apparent structural damage. (Wood, 1987)

Vina Del Mar

The damage observed in the resort city of Vina Del Mar, where the strong ground motions were recorded was reported light, except for a few unique cases. Less than 4% of residences suffered major damage. No deaths were recorded as a result of collapsing buildings. The structural characteristics of this region are different from that of the U.S. The buildings above five stories constructed from reinforced concrete, 95% of buildings with five or more stories, typically had a large portion of the floor area, approximately 6%, dedicated to structural walls dedicated to resisting lateral loads. Due to this fact and from previous earthquake experience, these wall are very lightly reinforced and no special details for ductility are required by the building codes. However, the performance of these buildings and the entire building inventory in Vina Del Mar was condidered very good considering the large magnitude of the earthquake and the severe ground motions in the immediate area. Experience with these structures has led to the conclusion that careful detailing and construction inspection are, in most cases, unnecessary. (Riddell, 1987)

Edificio El Faro, located in Reneca, a suburb of Vina Del Mar suffered severe damage tilting to one side. It did not collapse, but resulted in the demolition of the building by dynamite after the event. Differential settlement in the Reneca region also caused reinforced concrete structure to break into two distinct sections. This was caused by compaction of sand and architectural configurations with the building constructed on a sloped grade. In the subdivision of Canal Beagle in Vina Del Mar a large distribution of localized damage was realized and attributed to unique conditions of terrain amplification.

San Antonio

The damage distribution seen around Vina Del Mar and the other developed regions showed a direct correlation with soil types and site conditions where engineered structures were built. This was especially realized in San Antonio the severity of damage increased due to soil liquefaction. In Cartagena, only 7km to the north of San Antonio, structures built on rocky sub-surfaces survived. A same situation occurred in Calparaiso, where damage was localized to an old river bed area. Again the buildings built on more solid ground in the hills performed well.

Comparison to the 2010 Chile Earthquake

The February 27 2010 Offshore Maule, Chile earthquake was one of the most significant seismic events in Chilean history concerning the dynamic behavior of tall buildings and previous earthquakes in the region. This earthquake also was a result of subduction of the Nazca plate under the South American plate. The epicenter was located95 km northwest of Chillan, Chile, 105 km north–north-east of Concepción, 115 km west– south-west of Talca and 335 km south-west of the capitol of Santiago. The earthquake left 486 dead compared to the 176 in 1985. The total estimated damage was estimated at $30 billion (US). This is considerably more than the $1 billion in 1985 even considering inflation. (Marshall, 2010)

The Chilean Public and design community is well aware of the seismic threat of the region and understands the strong earthquakes of the past including the large earthquake of 1985. This awareness is far greater than that in the US and they have taken great measures to ensure adequate seismic performance in buildings. (Marshall, 2010)

The seismic design provisions and building codes have been developing in recent years with influence from the earthquake design provisions of the West Coast of the US. The official Chilean building code (NCh433.Of96) is the most current code dated 1996 and issued by the National Institute of Normalization (1996). It is highly based on the US codes of the time and reinforced concrete design has been based on ACI 318 Requirements for Structural Concrete. According to the Los Angeles Tall Buildings Structural Design Council (LATBSDC), who met with design professionals from Chile, the level of structural engineering practice in Chile is sophisticated and includes a high level of peer review of structural designs. (Marshall, 2010)

Conclusions

The March 3, 1985 Chile Earthquake ranks with some of the largest seismic events in Chile history. These events including the most recent 2010 Maule earthquake are due to the subduction of the Nazca plate beneath the South American plate, which is active and will inevitably cause future events challenging professional design theory and the integrity of building construction in central Chile. Damage from the earthquake ranged from none to severe with extenuating circumstances of certain regions. Many of the localized damages have been attributed to site soil conditions, which acts as a learning experience for new structural designs.

Comparing the entire building inventory across central Chile describes a structural engineering practice that had adapted over the years based on previous experiences. The overall good performance of engineered structures including tall buildings is a testament to the awareness of the seismicity of the region. This awareness has developed into a philosophy that embraces worldwide seismic performance including the consideration and embrace of more advanced building codes comparable to that of earthquake design on the West Coast of the US. As experienced in 2010 earthquake the building inventory once again performed well, however Chilean officials will continue to adapt and improve design techniques to large seismic events in an efficient, economical, and practical manner.

Technical Report:Contains 8 separate articles relating to the 1985 earthquake. Articles discuss everything from the site conditions and seismicity of the area (as well as field tests run to quantify these discussions), to the reported spectra for the area, analysis of overall ground response and building performance. Provides records of each earthquake shock, as well as energy release records. Discusses building performance by location and includes evaluation of damage.

Journal Article:Edmund Booth spent six days in Chile with Earthquake Engineering Field Investigation Team whose goal was to study surviving structures to isolate favorable and unsuccessful features of the structural systems. A background of the day of and characteristics of the earthquake are presented. A history of typical Chilean building construction and earthquakes is discussed. Outcomes and consequences including fatalities and serious building damages are covered as well. The article serves as a general overview of the event and aftermath.

Technical Report:Discusses observed behavior and resulting damages, resulting from the earthquake, of over 400 reinforced concrete buildings in Vina Del Mar between 5 and 23 stories. The various observed behaviors and damage is then interpreted in relations to physical characteristics of the corresponding structural systems. Damage statistics were compared through structural indicies based on floor areas, structural wall and column areas, and mass. Structural and nonstructural damages were considered.

Technical Report:Discusses geology and seismicity of the Vina Del Mar region, development of seismic provisions in Chile in the last 100 years, and ground motion records from the earthquake. Typical construction and design methods are described for concrete and masonry buildings in the area Thirteen reinforced concrete buildings in Vina Del Mar and Valparaiso were visited and documented extensively and described in the report. The descriptions included building location, structural designs and layout, observed damages, and estimated cost of repairs.

Additional Resources

Periodical:takes a personal look at the 1985 earthquake. Discusses the countless people that were left to sleep in parks and on sidewalks because their homes were damaged or destroyed by the earthquake.

Journal Article:Attempts to find a pattern between the 1985 earthquake and those that took place earlier in the same area. Looks at historical records, looks for correlation in earthquake return period, but unsuccessful due to the varying nature of each earthquake that hits the Chile region. Concludes that the peculiar predictability of Chilean earthquakes is the only similarity.

Journal Article:Provides background information on the area’s seismic activity and recounts information about the 1985 earthquake. Discusses immediate consequences of the earthquake to buildings, people, infrastructure and schools. Also discussions the reaction of the population- which was to help, rather than take advantage of the chaotic situation. Additionally, discusses injuries and groups those injured statistically, and by injury type. Does the same for those who died. Generally an all-encompassing source that looks at the human aspect of the 1985 earthquake.

Journal Article:Discusses the complete failure of a 8-story reinforced concrete building that experienced brittle failure on the first floor. The peculiarity is that it was the only structure constructed and designed in that way that failed so catastrophically. The article analyzes the building’s properties and theoretical response and then compares with the data collected from the actual ground motion. Concludes that the likely cause of collapse was the failure of a structural detail.